Two-dimensional molybdenum diselenide nanosheets as a novel electrode material for symmetric supercapacitors using organic electrolyte

Abstract

Two-dimensional transition metal chalcogenides have gained much consideration as electrode materials in electrochemical energy storage devices. In this work, we successfully prepared 2H-MoSe₂ sheets and investigated their charge-storage performance in organic electrolyte via fabrication of symmetric supercapacitor (SSC). The formation of 2H-MoSe₂ nanosheets was confirmed using X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscope, Raman spectrum and mapping analyses, respectively. The cyclic voltammetric analysis revealed the presence of pseudocapacitive nature of charge-storage in the MoSe₂ SSC with a specific cell capacitance of 25.31 F g⁻¹ obtained at a scan rate of 5 mV s⁻¹. The charge-discharge analysis revealed that the MoSe₂ SSC possesses a high specific cell capacitance of 16.25 F g⁻¹ (obtained at a current density of 0.75 A g⁻¹), an energy density of 20.31 Wh kg⁻¹ and excellent cyclic stability with capacitance retention of about 87% over 10,000 cycles. The MoSe₂ SSC delivered an excellent power density of 7.5 kW kg⁻¹ obtained from the CD profiles measured using a current density of 5 A g⁻¹. The energy/power density of the MoSe₂ SSC device is comparable or even higher with the reported SSCs using 2D materials such as graphene sheets, siloxene sheets, and MXene sheets, respectively. Electrochemical impedance spectroscopic analysis (Nyquist and Bode plots) were used to understand the capacitive nature and charge-transfer kinetics of the MoSe₂ SSC in organic electrolyte. Furthermore, we have also demonstrated the real-time application of the MoSe₂ SSC as an indication of their candidature towards the development of next-generation energy storage devices.

Introduction

Two-dimensional transition metal chalcogenides (TMCs) received increasing attention in the field of energy harvesting and energy storage sectors due to their electronic conductivity, optical properties, and distinct electrochemical properties [[1], [2], [3]]. Recently, 2D TMCs based on sulfides such as vanadium disulfide (VS₂), titanium disulfide (TiS₂), and molybdenum disulfide (MoS₂) has demonstrated as potential candidates for use as electrode materials for supercapacitors, batteries, and hybrid ion capacitors [4,5]. The merits of 2D-TMCs over transition metal oxides and binary metal oxides for energy storage devices is their high electronic conductivity with surface redox properties and high ionic diffusivity compared to their oxide analogues [6]. Increasing efforts are made towards the preparation of single or few layered 2D TMCs via chemical methods as well as exfoliation methods during this decade [7]. The mechanism of charge storage in layered TMCs can be due to (i) electrical double layer capacitance, (ii) pseudocapacitance, and (iii) ion-intercalation capacitance which strongly depends on the electronic conductivity, thickness/layer numbers, and lateral size of the 2D TMCs [[8], [9], [10]]. In our earlier study, the specific capacitance of bulk MoS₂ was increased nearly five-fold after exfoliation into few layered MoS₂ nanosheets in LiOH electrolyte [11]. Due to these interesting properties, the research in the development of TMCs based supercapacitors gained higher attention in this decade. Hitherto, TMCs based on metal sulfides are extensively studied compared to the metal selenides in which the latter is expected to deliver excellent electrochemical properties due to their large anionic polarizability (as well as high ionic diffusivity) arises from the Se²⁻ as similar to that of sulfides [12,13]. Up-to-date, the studies on the electrochemical performances of metal selenides are very limited and only a few reports available in the literature. Therefore, the study on the electrochemical performances of TMCs based on selenides is highly important.

Considering the efforts taken on the metal selenides based electrodes, the performances of cobalt selenides, nickel selenides, and molybdenum selenides has been reported recently [[13], [14], [15], [16]]. Among these materials, MoSe₂ is meritorious due to its sheet-like structures with Se-Mo-Se individual nanosheets separated via van der-Waals interactions similar to that of MoS₂, thus providing sufficient space for ion intercalation and de-intercalation process [17]. Recently, researchers focused on understanding the electrochemical properties of MoSe₂ sheets for applications in batteries and supercapacitors [[18], [19], [20]]. At first, Carmen et al. demonstrated the enhanced specific capacitance of exfoliated MoSe₂ (17 F g⁻¹) compared to bulk MoSe₂ (∼3 F g⁻¹) using a three-electrode configuration [20]. Later on, Bissett et al. demonstrated the performance of MoSe₂ symmetric supercapacitor (without conductive additive) with a specific capacitance of 2.57 F g⁻¹ [5]. In order to improve the energy storage properties of MoSe₂, researchers carried out different strategies such as developing binder-free electrodes and obtaining high surface area MoSe₂ with different morphologies. For instance, Huang et al. demonstrated the enhanced electrochemical properties of hydrothermally grown MoSe₂/Ni foam with a specific capacitance of 1114 F g⁻¹ [21]. Further, they also demonstrated the use of carbon materials to increase the specific capacitance of MoSe₂ such as MoSe₂/graphene and porous layered MoSe₂/acetylene black binder-free electrodes which delivered a specific capacitance of 1422 and 2020 F g⁻¹, respectively [15,22]. Gao et al. reported a specific capacitance of 243 F g⁻¹ for the hydrothermally prepared MoSe₂ nanosphere based electrodes [23]. In our recent work, we demonstrated the electrochemically deposited MoSe₂ sheets with a specific capacity of 548 mAh g⁻¹ (specific capacitance of about 2468 F g⁻¹) [24]. These studies demonstrated the use of MoSe₂ electrode for supercapacitors examined using a three-electrode system. However, two-electrode tests are more reliable for elucidating the materials properties and device performance [25]. A previous study reported that the MoSe₂ based symmetric supercapacitor device (0.5 M  H₂SO₄ electrolyte) delivered a specific capacitance of 10.4 F g⁻¹ at a current density of 0.1 A g⁻¹ [26]. Recently, Qiu et al. reported MoSe₂ based solid state supercapacitor (using MoSe₂ rods and sheets as two electrode) with enhanced energy-storage properties due to the high operating potential window of 1.4 V [27]. These studies suggested that the morphology, electrode fabrication, and surface area of the MoSe₂ highly reflects on their electrochemical capacitive properties.

Another promising way to enhance the energy density of the supercapacitor is the use of ionic or organic electrolytes which can provide a wide voltage window of nearly 3.5 V, thus resulting in the improved energy density of the supercapacitor [28]. For instance, the recent work of Suarez et al. demonstrated that the energy density of graphene-based symmetric supercapacitor is increased from 5 to 16 Wh kg⁻¹ from aqueous to ionic liquid electrolyte [29]. Therefore, a study of capacitive properties of MoSe₂ using ionic liquid electrolyte will result in enhanced energy storage properties. Up to date, the electrochemical properties of MoSe₂ nanosheets are examined only in aqueous electrolytes and gel polymer electrolytes. However, the study on the electrochemical properties of MoSe₂ nanosheets in non-aqueous organic electrolytes is not yet studied. In this study, the electrochemical properties of MoSe₂ nanosheets in tetraethylammonium tetrafluoroborate/acetonitrile (TEABF₄/AN) electrolyte was investigated using the fabrication of symmetric supercapacitor (SSC).